Does 'olfactory white' smell a bit familiar?

Some mixtures of 30-plus ingredients start to smell the same, researchers find

ScienceNOWDecember 9, 2012 

Noam Sobel, right, is a neuroscientist at the Weizmann Institute of Science in Rehovot, Israel.

If you play sounds of many different frequencies at the same time, they combine to produce neutral “white noise.” Neuroscientists say they have created an analogous generic scent by blending odors.

Such “olfactory white” might rarely, if ever, be found in nature, but it could prove useful in researching the sense of smell.

Using just a few hundred types of biochemical receptors, each of which respond to just a few odorants, the human nose can distinguish thousands of different odors. Yet humans can’t easily identify the individual components of a mixture, even when they can identify the odors alone, says Noam Sobel, a neuroscientist at the Weizmann Institute of Science in Rehovot, Israel.

Now, he and his colleagues suggest, various blends made up of a large number of odors all begin to smell the same – even when the blends share no common components.

For their study, the researchers used 86 nontoxic odorants that had a wide variety of chemical and physical properties such as molecular structure, molecular weight and volatility.

Those chemicals also spanned a perceptual scale from “pleasant” to “unpleasant” and another such scale on which scents were judged to range from “edible” to “poisonous.”

The researchers then diluted the chemicals so that their odors were equally intense.

Finally, they created mixtures by dripping individual odorants onto separate regions of an absorptive pad in a jar, a technique that prevented the substances from reacting in liquid form to create new substances or odors. The odor blends contained anywhere from one to 43 of the chemicals, Sobel said.

In the tests, volunteers sniffed a mixture and then compared it with other mixtures made up of varying numbers of odorants. When the test mixture had just a few components, volunteers could easily distinguish it from the other blends, Sobel said.

But as the number of odorants in a mixture rose above 20, volunteers began to perceive the blends as becoming more and more similar.

By the time mixtures contained 30 or more components, most of the blends were judged to smell alike, the researchers report online in the Proceedings of the National Academy of Sciences.

The team dubbed the generic scent associated with large mixtures “olfactory white.”

Although many scents – such as coffee, wine, roses and dirty socks – are complex blends containing hundreds of components, they are very distinctive.

At least two factors are responsible, Sobel said: The individual odorants are often chemically related, and often one or more of them is more intense than the rest.

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